The present invention relates generally to a vapor generator for use in vapor deposition equipment. In particular, the present invention relates to a vapor generator designed for the requirements of vapor phase epitaxy and other chemical vapor deposition equipment.
Group III-V compound semiconductor materials including different monocrystalline layers with varying compositions and with thickness ranging from fractions of a micron to a few microns are used in the production of many electronic and optoelectronic devices such as lasers and photodetectors. Chemical vapor deposition methods using organometallic compounds are typically employed in the chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) art for the deposition of metal thin-films or semiconductor thin-films of Group III-V compounds. Compounds typically used as precursors in CVD for the semiconductor industry include cyclopentadienyl magnesium (xe2x80x9cCp2Mgxe2x80x9d), trimethyl aluminum (xe2x80x9cTMAxe2x80x9d), trimethyl gallium (xe2x80x9cTMGxe2x80x9d), triethyl gallium (xe2x80x9cTEGxe2x80x9d), trimethyl antimony (xe2x80x9cTMSbxe2x80x9d), dimethyl hydrazine (xe2x80x9cDMHyxe2x80x9d), trimethyl indium (xe2x80x9cTMIxe2x80x9d) and the like. Solid precursors, such as TMI, are used in the metal-organic-vapor-phase-epitaxy (xe2x80x9cMOVPExe2x80x9d) of indium containing semiconductors.
Typically, such solid precursors are placed in a cylindrical vessel or container referred to as a xe2x80x9cbubblerxe2x80x9d and subjected to a constant temperature wherein the solid precursor is vaporized. A carrier gas, such as hydrogen, is employed to pick up the precursor compound vapor and transport it to a deposition system. Most solid precursors exhibit poor and erratic delivery rates when used in conventional bubbler-type precursor vessels. Such conventional bubblers include both bubbler vessels having a dip-tube attached to the inlet, see for example U.S. Pat. No. 4,506,815 (Melas et al.), or the gas-feeding device as disclosed in U.S. Pat. No. 5,755,885, which has a plurality of gas-ejecting holes in the dip-tube to introduce the carrier gas into the container. Such conventional bubbler systems can result in a non-stable, non-uniform flow rate of the precursor vapors, especially when solid organometallic precursor compounds are used. Non-uniform flow rates produce an adverse affect on the compositions of the films, particularly semiconductor films, being grown in MOVPE reactors.
Other bubbler systems have been developed, such as that developed by Morton International, Inc., which eliminates the use of a dip-tube. However, while such dip-tube free bubblers were found to provide a uniform flow rate, they failed to provide a consistently high concentration of precursor material. The inability to achieve a stable supply of feed vapor from solid precursors at a consistently high concentration is problematic to the users of such equipment, particularly in semiconductor device manufacture. The unsteady organometallic precursor flow rate can be due to a variety of factors including progressive reduction in the total surface area of chemical from which evaporation takes place, channeling through the solid precursor compound where the carrier gas has minimal contact with the precursor compound and the sublimation of the precursor solid material to parts of the bubbler where efficient contact with the carrier gas is difficult or impossible.
Various methods have been adopted to overcome the flow problems such as the use of revers flow bubblers, the use of dispersion materials in the precursor materials, employing diffuser plates beneath the bed of solid precursor material, employing conical cylinder designs and beating on the cylinder to de-agglomerate the solid precursor material. For example, U.S. Pat. No. 4,704,988 (Mellet) discloses a bubbler wherein the vessel is separated by a porous partition into first and second compartments. In this design, the precursor material is contained in the first compartment in a liquid state and when vaporized diffuses through the partition into the second compartment where it contacts and is entrained in a carrier gas for transport from the vessel into the appropriate deposition chamber.
U.S. Pat. No. 5,603,169 (Kim) discloses a bubbler design having lower and upper porous plates through which the carrier gas passes. The lower porous plate is located above the carrier gas feed inlet and supports the solid precursor material. In operation, carrier gas passes through the lower porous plate before contacting the solid precursor material. A compressing plate is located above the lower porous plate for pressing the precursor material by its weight. Such bubbler design is quite complex and suffers from a problem of fluidizing the solid precursor material due to carrier flow through the porous plug before passing upward, i.e. against gravity, through the bubbler. This causes changes in the effective area of the solid precursor material which adversely affects the performance of the bubbler.
Conventional bubbler designs fail to provide a uniform flow rate with maximum pick-up of precursor material. There is thus a continuing need for stable flow/pick-up of solid precursor material vapor. Further, there is a need for bubbler devices that are tailored to provide a uniform and high concentration of precursor material vapor until total depletion of the vapor source.
It has been surprisingly found that the bubbler designs of the present invention provide a stable flow rate of precursor material vapor, provide a high concentration of precursor vapor in the carrier gas, can be used at lower pressures than conventional bubblers, and provides maximum contact of the carrier gas with the precursor material.
In one aspect, the present invention provides a device for providing vaporized organometallic compound to a chemical vapor deposition system including a vessel having an elongated cylindrical shaped portion having an inner surface defining a substantially constant cross-section throughout the length of the cylindrical portion, a top closure portion, a bottom closure portion, and inlet and outlet chambers in fluid communication and separated by a porous element, the top closure portion having a fill plug and a gas inlet opening, the fill plug and gas inlet opening communicating with the inlet chamber, the outlet opening communicating with the outlet chamber, the inlet chamber having a conical shaped lower portion containing the porous element, the porous element being spaced from the bottom closure portion.
In a second aspect, the present invention provides a method for providing organometallic precursor compound in the vapor phase to a chemical vapor deposition system including the steps of: a) introducing an organometallic precursor compound into the device described above; b) heating the organometallic precursor compound; c) passing a carrier gas through the organometallic precursor compound to provide a gas stream containing vaporized organometallic precursor compound; and d) delivering the gas stream to a chemical vapor deposition system.
In a third aspect, the present invention provides an apparatus for chemical vapor deposition of an organometallic precursor compound including the device described above.